I have two layers (layer 0 and layer 1). 1 emitter on one layer, another (different) emitter on second layer. Yet I can't see both emitters / layers at the same time.
Is there an option for turning on all layers or something ?
Check to make sure nothing is turned off in the Layers window. Unless you have the emitters or particles hidden on a layer (either by turning them off in the Layers window or by clicking the star icon beside the emitter's name in the Hierarchy window), everything on both layers should be visible.
Unless you mean that you want to see the emitter symbols and paths on all of the layers at the same time. If that's what you mean, then r-click (control-click on one-button Mac) in the Layers window and uncheck "current objects only".